Groundwater elevations and temperature adjacent to a beaver pond in central Oregon

Abstract:

This study was designed and implemented to observe
the spatial and temporal dynamics of groundwater levels
and temperatures adjacent to a beaver pond in semi-arid
central Oregon. The study site was located on the eastern
boundary of Painted Hills National Monument along Bridge
Creek, a tributary to the John Day River. Groundwater
levels and groundwater temperature were monitored in 64
wells from July 3, 1991 to June 11, 1992.
Groundwater elevations varied seasonally and were
generally positively correlated with increased streamflow.
In addition, beaver dam-building activity appeared to
increase aquifer recharge near the beaver pond in
comparison to downstream areas. The groundwater elevation
of a well located near the pond rose 0.35 m between August
and November 1991, while the Beaver pond stage increased
by 0.22 m. Groundwater elevation at another well located
downstream of the dam increased by only 0.17 in during this
period with a corresponding increase in stream stage of
0.05 in. Groundwater levels throughout the study site
averaged a 0.31 in gain from August to November 1991.
All wells at the study site responded to changes in
streamflow, and thus appear to be hydraulically connected
to the stream.
Hydraulic gradients adjacent to the pond were
relatively low (i.e. 0.005 m/m). However, a zone about 50 m from the pond of relatively high hydraulic gradient
(0.05 m/m) persisted over time, and groundwater flow
directions in this area were both normal and parallel to
the stream. Based on hydraulic gradients and observed
groundwater elevations, the movement of water from the
stream to subsurface recharge of riparian areas appeared
to be greater near the pond than the streamside locations.
Approximately 90 m³ of water could be drained from
the aquifer if the dam were breached. Results of this
study support the conclusion conmmonly expressed in the
literature, but seldom quantified, that elevated water
tables occur adjacent to beaver ponds.
Groundwater temperatures for each successive month
closely followed stream temperatures in wells next to the
stream, indicating that stream temperatures readily
influence groundwater temperatures adjacent to the stream.
Wells located relatively far out on the floodplain
(i.e. 50 meters) from the beaver pond had about a two
month temperature response lag behind stream temperature.
These results further indicate that stream temperature can
influence groundwater temperature, and that groundwater
recharge is highest near the pond.